In a wireless communication system, a user equipment performs radio link monitoring by measuring the state of a downlink channel connected to the current base station, evaluating the downlink quality based on measurement results, and determining whether the downlink channel is able to provide a reliable level of service. As a representative example, the Long Term Evolution (LTE) system, which is developed as a next generation mobile communication system by the 3rd Generation Partnership Project (3GPP), a standardization body for asynchronous cellular mobile communication, specifies that a user equipment should evaluate downlink quality by measuring a common reference signal (CRS) transmitted on downlink from a corresponding base station for radio link monitoring. Here, reference signals that a user equipment may receive on the downlink of the LTE system may include the CRS described above, channel state information reference signal (CSI-RS), and demodulation reference signal (DMRS).
To evaluate radio link quality, the user equipment performs CRS-based measurement for a given time, filters measurement values, and compares the filtered results with preset thresholds. Here, the threshold is defined as the signal level corresponding to a given block error rate (BLER) of a Physical Downlink Control Channel (PDCCH) transmission distributed over the entire downlink channel bandwidth. In the current specification, the threshold Q_out is defined as the level corresponding to a block error rate of 10 percent, and the threshold Q_in is defined as the level corresponding to a block error rate of 2 percent. The user equipment sets thresholds Q_out and Q_in respectively to the levels corresponding to the given block error rates of a hypothetical PDCCH transmission taking into account Physical Control Format Indicator Channel (PCFICH) errors, and compares the estimated level with the thresholds.
When the level of the estimated radio link quality is lower than the threshold Q_out, the current radio link quality is evaluated as “out-of-sync”; and when the level of the estimated radio link quality is higher than the threshold Q_in, the current radio link quality is evaluated as “in-sync”. “Out-of-sync” indicates that the current downlink state is not reliable, and “in-sync” indicates that the current downlink state is reliable. According to evaluation results, the physical layer of the user equipment sends either an out-of-sync indication or an in-sync indication to the higher layers.
When the out-of-sync indication occurs consecutively a given number of times or more, the higher layer of the user equipment detects a downlink quality error and switches to a physical layer problem detection state, in which a corresponding timer is started. When the user equipment does not recover from the physical layer problem detection state before the timer expires, radio link failure is declared. When the in-sync indication occurs consecutively a given number of times or more at the physical layer before the timer expires, the user equipment recovers the normal state from the physical layer problem detection state.
In 3GPP Release 11 currently under standardization, it is proposed to introduce a new carrier type without CRS transmission. Hence, the existing CRS-based scheme for radio link monitoring may no longer be applicable.
In particular, for a carrier of the new type, it is highly probable to configure data and control channels through frequency division multiplexing (FDM). Hence, it may be possible to provide a downlink service to a user equipment even when only a frequency range within the downlink channel bandwidth on a carrier of the new type exhibits a reliable level of quality.
In such a situation, when Q_out and Q_in are defined with reference to given block error rates of a hypothetical PDCCH transmission distributed over the overall downlink channel bandwidth as in the conventional manner, if the average quality of the overall downlink channel bandwidth is lower than Q_out, the radio link quality would be evaluated as out-of-sync although a frequency range within the downlink channel bandwidth exhibits a level of quality sufficient to maintain a downlink service. Likewise, although a frequency range within the downlink channel bandwidth exhibits a level of quality evaluating to in-sync, the radio link quality would not be evaluated as in-sync because the average quality of the overall downlink channel bandwidth is lower than Q_out.
For example, in the event that the existing radio link quality evaluation scheme is applied to the new carrier type, a situation may arise as shown FIG. 1 wherein Q_out is set as indicated by indicia 100 and the average quality of the overall downlink channel bandwidth is estimated by the user equipment as indicated by indicia 101. Here, although frequency ranges which exhibit a level of channel quality higher than Q_out and are usable for allocating control and data channels are present within the downlink channel bandwidth as indicated by indicia 102, the user equipment will evaluate the current radio link quality to out-of-sync.
A user equipment residing at a cell edge where the average level of quality of the overall downlink channel bandwidth is close to the threshold is highly likely to encounter the above situation. Thereby, although it is possible for the user equipment to receive a service from the current cell, the user equipment may perform an undesired operation such as conducting unnecessary handover to a neighbor cell, entering the physical layer problem detection state, or entering the radio link failure state.
Hence, it is necessary to develop a new technique that can solve the above problems and support smooth radio link monitoring.